The vortex correlation along the c axis in high quality single crystals of YBa2Cu4O8 has been investigated as a function of temperature T in different magnetic fields, using the flux transformer configuration. ii simultaneous sharp drop associated with the vortex lattice melting is observed in both the primary and secondary voltages (V-top and V-bot) Just above the melting temperature, the vortices form three-dimensional line Liquid with the correlation length along the c direction L(c)less than or equal to t, the sample thickness. The temperature where a resistive peak in R-bot develops corresponds to the decoupling temperature T-d at which the vortices lose their correlation along the c direction and they dissolve into the two-dimensional pancake vortices. The H-T phase diagram for the YBa2Cu4O8 single crystal is obtained.